The present invention relates to a process for the electrochemical roughening of aluminum which can be used for printing plate supports, said process being performed by means of alternating current in an aqueous mixed electrolyte.
Printing plates (this term referring to offset-printing plates, within the scope of the present invention) usually comprise a support and at least one radiation-sensitive (photosensitive) reproduction layer arranged thereon, the layer being applied to the support either by the user (in the case of plates which are not pre-coated) or by the industrial manufacturer (in the case of pre-coated plates). As a layer support material, aluminum or alloys thereof have gained general acceptance in the field of printing plates. In principle, it is possible to use these supports without modifying pretreatment, but they are generally modified in or on their surfaces, for example, by a mechanical, chemical and/or electrochemical roughening process (sometimes also called graining or etching in literature), a chemical or electrochemical oxidation process and/or a treatment with hydrophilizing agents. In modern continuously working high-speed equipment employed by the manufacturers of printing plate supports and/or pre-coated printing plates, a combination of the afore-mentioned modifying methods is frequently used, particularly a combination of electrochemical roughening and anodic oxidation, optionally followed by a hydrophilizing step. Roughening is, for example, carried out in aqueous acids, such as aqueous solutions of HCl or HNO.sub.3 or in aqueous salt solutions, such as aqueous solutions of NaCl or Al(NO.sub.3).sub.3, using alternating current. The peak-to-valley heights (specified, for example, as mean peak-to-valley heights R.sub.z) of the roughened surface, which can thus be obtained, are in the range from about 1 to 15 .mu.m, particularly in the range from 2 to 8 .mu.m. The peak-to-valley height is determined according to DIN 4768, in the October 1970 version; the peak-to-valley height R.sub.z is the arithmetic mean calculated from the individual peak-to-valley height values of five mutually adjacent individual measurement lengths.
Roughening is, inter alia, carried out in order to improve the adhesion of the reproduction layer to the support and to improve the water acceptance of the printing form which results from the printing plate upon irradiation (exposure) and developing. By irradiating and developing (or decoating, in the case of electrophotographically-working reproduction layers), the ink-receptive image areas and the water-retaining non-image areas (generally the bared support surface) in the subsequent printing operation, are produced on the printing plate, and thus the actual printing form is obtained. The final topography of the aluminum surface to be roughened is influenced by various parameters, as is explained by way of example in the text which follows:
The paper "The Alternating Current Etching of Aluminum Lithographic Sheet", by A. J. Dowell, published in Transactions of the Institute of Metal Finishing, 1979, Vol. 57, pages 138 to 144, presents basic comments on the roughening of aluminum in aqueous solutions of hydrochloric acid, based on variations of the following process parameters and an investigation of the corresponding effects: The electrolyte composition is changed during repeated use of the electrolyte, for example, in view of the H.sup.+ (H.sub.3 O.sup.+) ion concentration (measurable by means of the pH) and in view of the Al.sup.3+ ion concentration, with influences on the surface topography being observed. Temperature variations between 16.degree. C. and 90.degree. C. do not show an influence causing changes until temperatures are about 50.degree. C. or higher, the influence becoming apparent, for example, as a significant decrease in layer formation on the surface. Variations in roughening time between 2 and 25 minutes lead to an increasing metal dissolution with increasing duration of action. Variations in current density between 2 and 8 A/dm.sup.2 result in higher roughness values with rising current density. If the acid concentration is varied in a range from 0.17 to 3.3% of HCl, only negligible changes in pit structure occur between 0.5 and 2% of HCl, whereas below 0.5% of HCl, the surface is only locally attacked and at the high values, an irregular dissolution of Al takes place. An addition of SO.sub.4.sup.2- ions or Cl.sup.- ions in the form of salts (e.g. by adding Al.sub.2 (SO.sub.4).sub.3 or NaCl) can also influence the topography of the roughened aluminum. Rectification of the alternating current shows that, obviously, both half-wave types are necessary to obtain a uniform roughening.
Thus, it can be assumed that the use of aqueous HCl solutions as electrolyte solutions for the electrochemical roughening of support materials made of aluminum is known in principle. With these solutions it is possible (as is also evidenced by a great number of commericially available printing plates) to achieve a uniform graining, which is particularly suitable for applications in the field of lithography, and the roughness values of which vary within a range which in general is appropriate for practical use. For certain applications (for example, in the case of certain negative-working reproduction layers) there is, however, required a uniform and relatively "flat" roughened surface topography, which is difficult to obtain in the known electrolyte solutions based on HCl, using modern, high-speed apparatus. For example, the process parameters must be kept within very narrow limits, and this involves a process which can only be controlled with great difficulty.
The influence of the electrolyte composition on the quality of roughening is, for example, also described in the following publications, in which aqueous mixed electrolytes are employed:
German Offenlegungsschrift No. 22 50 275 (British Patent Specification No. 1,400,918) specifies aqueous solutions containing from 1.0 to 1.5% by weight of HNO.sub.3 or from 0.4 to 0.6% by weight of HCl and optionally from 0.4 to 0.6% by weight of H.sub.3 PO.sub.4, for use as electrolytes in the roughening of aluminum for printing plate supports, by means of alternating current, PA1 German Offenlegungsschrift No. 28 10 308 (U.S. Pat. No. 4,072,589) mentions aqueous solutions containing from 0.2 to 1.0% by weight of HCl and from 0.8 to 6.0% by weight of HNO.sub.3 as electrolytes in the roughening of aluminum with alternating current, PA1 German Auslegeschrift No. 12 38 049 (U.S. Pat. No. 3,330,743) mentions, as additional components in aqueous HNO.sub.3 solutions used in the roughening of aluminum for printing plate supports with alternating current, protective colloids acting as inhibitors, for example, lignin, benzaldehyde, acetophenone or pine needle oil, PA1 U.S. Pat. No. 3,963,594 specifies aqueous solutions containing HCl and gluconic acid as electrolytes in the electrochemical roughening of aluminum for printing plate supports, and PA1 German Auslegeschrift No. 22 18 471 (U.S. Pat. No. 3,755,116) mentions the addition of anticorrosive agents, which include monoamines, diamines, carboxylic acide amides, urea, chromic acid and non-ionic surfactants, to an aqueous HCl electrolyte, for roughening aluminum suitable for printing plate supports. PA1 German Pat. No. 120,061, describing the use of alkali metal salts of hydrofluoric acid in the production of Al or Zn printing plate supports; PA1 German Pat. No. 695,182, describing the use of hydrofluoric acid or its salts in the production of bearing surfaces of pistons or cylinders of aluminum; PA1 German Offenlegungsschrift No. 14 96 825, describing the use of salts of fluoboric acid (HBF.sub.4) in an almost saturated solution for the anodic treatment of metallic workpieces; however, only the treatment of steel sheet is explicitly mentioned in this context. In a comparative example, NaF is employed; PA1 German Offenlegungsschrift No. 16 21 090 (British Patent Specification No. 1,166,901), describing the use of fluosilicic acid (H.sub.2 SiF.sub.6) in a mixture with water and ethylene goycol for etching special Be/Cu or Ni/Fe/P alloys; PA1 German Offenlegungsschrift No. 16 21 115 (U.S. Pat. No. 3,632,486 and No. 3,766,043), describing the use of aqueous hydrofluoric acid in the roughening of aluminum webs for decorative panellings or printing plates, whereby the aluminum is switched such that it forms the anode; PA1 German Auslegeschrift No. 24 33 491 (British Patent Specification No. 1,427,909), describing the use of fluorinated anion-active surfactants (for example, 2-perfluorohexyl-ethane-1-sulfonic acid) in addition to an acid, such as hydrocholoric acid, for producing a "lizard-skin-type" texture on the aluminum surface, under the action of alternating current, whereby the texture which can be achieved in this way is said to give the aluminum surface an attractive appearance; and PA1 Japanese patent application No. 17 580/80, describing the use of a mixture of hydrochloric acid and alkali metal halides in the production of aluminum printing plate supports, whereby the only halide used in the examples is NaCl. PA1 The products have a uniform surface topography, a property, by which both the stability of print runs which can be achieved using printing forms produced from this support material, and also the water acceptance during printing, are positively influenced. PA1 Compared with the use of electrolytes containing purely hydrochloric acid, "pitting" (pronounced depressions, compared to the roughening of the surrounding surface) occurs less frequently and can even be suppressed completely. PA1 These surface properties can be materialized without much equipment expenditure, and the properties can be achieved within a wide range of roughening intensities. PA1 Employing this process, surfaces roughened in a particularly slight and uniform manner can be achieved, which is not possible to the same degree using the known electrolytes. PA1 The mixed electrolyte used in the process of this invention is electrochemically stable, i.e., it does not decompose when high current loads (voltages) are applied.
The known organic additives to aqueous acid electrolytes, such as HCl or HNO.sub.3 solutions, have the disadvantage that, in the case of high current loads (voltages), they become electrochemically unstable in the modern continuously working web processing apparatus and decompose at least partially. The known inorganic additives, such as phosphoric acid, chromic or boric acid, exhibit the disadvantage that quite often there is a local breakdown of their intended protective effect, as a consequence whereof single, particularly deep pits are formed at the respective spots.
In general, the known complex-forming additives accelerate the dissolution of the aluminum due to their "trapping " of released Al.sup.3+ ions and thus cause an increased roughening action. As a result thereof, quite often no creation of new pores is initiated, but pores which are already existent continue to grow. i.e., increased pitting occurs. It is true that usually the growth of individual pores is stopped relatively soon by the known inhibiting additives, and the formation of new pores can be initiated. These inhibitors exhibit, however, the decisive disadvantage that this protective effect can collapse due to voids, alloy constituents, and the like, so that single pores which are too deep are obtained on an otherwise evenly and uniformly roughened surface. Support materials exhibiting this kind of defects are not suitable for lithographic purposes.
There have also been disclosed aqueous electrolyte solutions having a content of inorganic or organic fluorine compounds, which may be present alone or in combination with other components, or of hydrofluoric acid, respectively, for the roughening of aluminum. Examples of such disclosures are:
Neither the electrolytes mentioned in the above references, nor the other mixed electrolytes, based on aqueous HCl solutions, which have been disclosed so far, result in surfaces of a quality which, irrespective of the peak-to-valley heights to be achieved in each case, is expected from currently available printing plate support materials. The roughening structure of aluminum supports roughened in pure aqueous hydrofluoric acid is too heterogeneous, and similar results are observed in those cases where simple halides (chlorides, fluorides, and the like) are employed as admixtures with a hydrochloric acid electrolyte. So far, complex fluorine compounds have not been used for the roughening of aluminum; a lizard skin-type surface structure is not suited for lithographic purposes.